Single-sideband system for the transmission of speech



Patented Oct. 6, 1959 SINGLE-SHDEBAND SYSTEM FOR THE TRANSMISSION F SPEECH Frank De .lager and Johannes Anton Greefkes, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Applicationv October 24, 1955, Serial No. 542,305 Claims priority, application Netherlands October 25, 1954 6 Claims. (Cl. 179-155) The invention relates to a single-sideband system comprising a transmitter and a receiver for the transmission of speech, the transmitter comprising, as usual, a singlesideband generator controlled by the speech signals to be transmitted, the receiver comprising a single-sideband ldemodulator.

It is known that, as compared with other modulation methods, for example, amplitude modulation, frequency modulation, pulseaduration modulation and the like, the single-sideband modulation has considerable advantages. The frequency band required for the transmission is small, i.e. primarily equal to the frequency band for low-frequency signals. With unfavourable signal to noise ratios of for example 20 to l5 db at the receiver input a single-sideband communication system permits, moreover, a serviceable speech communication, so that a comparatively low transmitter power may suiiice. However, a single-sideband transmitter must be proportioned in accordance with the peak signal load and in contradistinction to a frequency-modulation transmitter it is normally not fully loaded which is a disadvantage.

The invention has for its object to obtain a high transmission eflciency in a single-sideband system for the transmission of speech in a manner such that on the one hand the transmitter is substantially fully loaded in normal operating conditions and that the signal-noise ratio at the receiver input may have a lower value than with the normal single-sideband system: thus with a given distance between transmitter and receiver a comparatively small transmission power permits of obtaining a practically serviceable speech communication.

To this end, in accordance with the invention, the transmitter of the single-sideband system of the aforesaid kind comprises a limiter coupled with the output of the single-sideband generator, the output signal of this limiter being `a single-sideband signal of substantially constant amplitude and, moreover, a single-sideband filter to suppress distortion products occurring in the limiter, the output signal of this single-sideband filter being transmitted to the receiver, in which it is supplied via a limiter to the single-sideband demodulator.

In order to improve the quality of the transmission of the speech signals, more particularly with respect to the l recognizability of voices, in the` transmitter the speech signals preferably are supplied, via a pre-emphasis net'- work to the single-sideband generator, the receiver then 'requiring a deemphasis network between the singlesideband demodulator and the reproducing device.

According to another aspect of the invention a further, very considerable improvement in the quality of the transmitted signals is obtained, both at favourable and unfavourable signal to noise ratios, by supplying in the transmitter the single-sideband signal on the one hand to l the limiter and on the other hand to an amplitude detector, followed by a low-pass filter to obtain a signal envelope, which contains at Vthe most the lowest of the speech frequencies to be transmitted and which, apart from the limited single-sideband signal, is transmitted to i;

was found experimentally that in some cases the signal to noise ratio of the output signal of the receiver then is more favourable than the signal to noise ratio after transmission of the limited single-sideband signal 4and of the signal envelope in frequency bands remote from' one another. y

When using the system described in the preceding paragraph, for example, for radio transmissions, in which the speech frequency band to be transmitted is about 0.3 to 3.4 kc./s. and the frequency band of the signal envelope may be, for example, 0 to 250 c./s., a speech communication of very satisfactory intelligibility is obtained with signal to noise ratios at the receiver input of only 12 to 9 db for the single-sideband signal, whilst with favourable signal to noise ratios of for example 40 db and more a quality is obtained which is normal for telephone communications.

At low signal to noise ratios of l5 db or less the signal to noise ratio occurring at the receiver output appears to be determined primarily by the signal to noise ratio of the pilot channel, so that for the transmission of the (unipolar) signal envelope narrow band frequency modulation (module about 2 to 3) is to be preferred over amplitude modulation. Moreover, a further restriction of the frequency band of the signal envelope to be transmitted is conducive to improve its signal to noise ratio. This frequency bandV may, for example, be`restricted to c./s. or even to 40 c./s. However, the latter reduction produces a slight reduction of the qualityY of the incoming speech signal at favourable signal to noise ratios at the receiver input.

The invention will be described Vmore fully with reference to the figures. Y

Figs. la and lb show a simple :embodiment of a transmitter and a receiver for a single-sideband systemaccording to the invention.

Fig. 2a shows an improved embodiment of a transmitter according to the invention and Fig. 2b shows a preferred variant of an embodiment of the channel for the signal envelope for use in the transmitter.

Fig. 3a lshows an improved embodiment of the receiver and Fig. 3b shows the channel for the signal envelope for use in the receiver corresponding to the signal envelope channel shown in Fig. 2b. v

'In the single-sideband transmitter shown in Fig. la the signals obtained from a microphone 1 are supplied via a pre-emphasis network 2 and a iilter 3, passing the desired speech frequency band, to a single-sideband generator 4. The single-sideband generator comprises Ya ring modulator 6, connected to a crystal-controlled carrier-wave oscillator 5, this modulator being followed by an amplifier 7, comprising, as usual, a single-sideband filter to select the desired sideband.

In accordance with the invention the single-sideband signal obtained from the single-sideband generator 4 is supplied to a limiter 8, which produces a limitation such that the output signal is a single-sideband signal of substantially constant amplitude. The limited single-sideband signal is supplied by way of a single-sideband lter 9, which serves to suppress the distortion products of the limiter, to a class C-amplier 10, to which a transmitter aerial 11 is connected. It has been found by experiments that the use of a single-sideband filter after the limiter does not produce an appreciable reduction of quality of the transmitted signal. y g The limiter 8-passes the single-sideband'compoxient having the largest input amplitude in a more satisfactory manner than the components having a smaller input amplitude. In order to improve the transmission quality it has been found to be favourable, with respect to this property of the limiter, to supply the speech signals from the microphone 1 by way of a pre-emphasis network to the single-sideband generator 4. The use of the pre-emphasis network 2 provides an amplitude-equalization of the single-sideband components, so that in the limited single-sideband signal the higher speech frequency components of a speech-frequency band of for example 0.3 to 3.4 kc./s. are fairly well represented.

The nal stages of the transmitter described above may be normally fully loaded by the limited single-sideband signal of substantially constant amplitude; thus, as is known with frequency-modulation transmitters, use may be made of class C-ampliers in the final stages of the transmitter.

Fig. lb shows the receiver to be used in conjunction with a transmitter shown in Fig. la. The signals obtained from the aerial 12 are supplied by way of a selective high-frequency pre-amplifier 13 to a limiter 14, which, like that of the transmitter, is constructed in a manner such that the limited single-sideband signal has a substantially constant amplitude. The limited singlesideband signal is supplied, in order to reduce the noise, preferably to a simple single-sideband lter 14 and then demodulated in the conventional manner by means of a single-sideband demodulator 15. The demodulator comprises a mixing stage 16, which is connected to a local carrier-wave oscillator 17. The low-frequency signals obtained are supplied through a filter 18, passing only the speech-frequency band of for example 0.3 to 3.4 kc./s., a de-emphasis network 19 and a final amplilier 20 to a reproducing device 21.

The use of the limiter 14 provides a material reduction of the interference of the incoming single-sideband signal; noise peaks are limited etectively and noise voltages slightly lower than the signal voltage are attenuated owing to the limiting efect of the limiter with respect to the peak portion of the signal voltage. The de-emphasis network 19 in the low-frequency circuit is, of course, required to compensate for the amplitude distortion produced in the transmitter by the pre-emphasis network.

The receiver shown in Fig. 1b may be constructed in a manner such that during the speech intervals the limiter is not completely driven by the noise then received. With such a proportioning of the limiter and the preceding amplifier(s) a certain dynamic range is retained in the transmitted speech channels, which is conductive to the intelligibilty.

An improved suppression of noise during the speech intervals may be obtained by adding in the transmitter, prior to the limitation, a pilot signal, which is weak with respect to the normal speech signals (for example to -20 db), to the single-sideband signal, for example in the position of the suppressed carrier wave. To this end the input of the limiter 8 of Fig. la is connected through an adjustable attenuator 7 to the carrier-wave oscillator 5. The output signal of the limiter then contains the pilot signal with an amplitude which is inversely proportional to the speech signal intensity. The pre-amplifier 13 then includes a single-sideband lter preceding the limiter 14 in the receiver which is constructed in a manner such that the pilot signal reaches the limiter with an amplitude suflicient for fully loading the limiter during the speech intervals.

With the last-mentioned transmitter construction the receiver to be described with reference to Fig. 3a'may also be used. Y

It has been found that the single-sideband system shown in Figs. la and lb permits of obtaining readily a speech communication at very low signal to noise ratios of for example v12 to 9 db at the receiver input.

4 I The quality of the transmitted speech, however, is materially lower than with the use of normal single-sideband transmission, particularly owing to the failing of dynamic signal properties.

In order to improve the system according to the invention in the last-mentioned respect, use may be made of transmitters and receivers of the kind shown in Figs. 2 and 3.

In the transmitter shown in Fig. 2a the speech signals obtained from a microphone 22 are supplied through a filter Z3, passing only the speech-frequency band of for example 0.3 to 3.4 kc./s., and a pre-emphasis network 24 to a single-sideband generator 25. The singlesideband generator 25 comprises a ring modulator 26, controlled by the speech signals and connected to a crystal-controlled carrier-wave oscillator 27. The modulated signals then obtained are supplied through an amplier 28, selecting the desired sideband, of the kind shown in Fig. la, to a limiter 29, which brings about a drastic limitation of the single-sideband signal supplied thereto. The limited single-sideband signal is supplied via a class C-amplier 30 and a single-sideband filter 31, serving to suppress the distortion products in the limiter, to a transmitter aerial 32.

The single-sideband signal obtained from the singlesideband generator 25 is also supplied to an amplitude detector 33, followed by a low-pass filter 34, in order to obtain a signal envelope, which contains at the most the lowest of the transmitted speech frequencies. With the aforesaid speech-frequency band of 0.3 to 3.4 kc./s. the cut-olf frequency of the low-pass filter 34 may, for example, be equal to 300 c./s. or less to 40 c./s.). The signal envelope thus obtained controls an amplitude modulator 35, connected to a pilot signal oscillator 36. The modulated pilot signal is supplied through an amplifier 37 and a lter 38, adapted to the bandwidth of the amplitude modulated pilot signal, also to the transmitter aerial 32.

The pilot signal oscillator 36 may, for example, be detuned by 3.7 to 4 kc./s. with respect to the carrier-wave oscillator 27, in a manner such that the pilot signal modulated in amplitude by the envelope is emitted in a frequency band adjacent that of the limited single-sideband signal from the channel 29, 30, 31. As an alternative, the pilot signal oscillator and the carrier-Wave oscillator 27 may be tuned to nearly the same or even equal frequencies. In the latter case the carrier-wave signal to be supplied to the modulator 35 may be obtained not from the pilot signal oscillator 36 but from the carrier-wave oscillator 27. The modulated pilot signal is then emitted with a frequency corresponding to the suppressed carrierwave of the single-sideband channel 29, 30, 31.

-It should especially be noted that the signal obtained from the envelope detector 33 contains a considerable direct-current component and is unipolar. It is advantageous to have the pilot signal modulated by the unipolar envelope, in a manner such that in the absence of speech signals the pilot signal is emitted at the maximum amplitude, which decreases at increasing intensity of the speech signals. This is favourable to suppress noise during speech intervals, as will be described more fully hereinafter with reference to the receiver shown in Fig. 3a.

It has been found that in the .transmission of singlesideband signals by means of a transmitter of the kind shown in Fig, 2a and a corresponding receiver, the signal to noise ratio of the incoming signals, particularly if the signal to noise ratios in the transmission path are unfavourable, for example lower than l5 db, is primarily determined by the signal to noise ratio in the transmission channel of the signal envelope.

It is, in this respect, favourable to construct the envelope channel in a manner such that at low signal to noise ratios an optimum signal to noise ratio of the received envelope is obtained. This may be obtained, not only by restriction of the band-width of the envelope AA. A

channel, but also by replacing in the transmitter shown in Fig. 2a the amplitude-modulation envelope channel 33 to 38 by a frequency-modulation envelope channel as shown in Fig. 2b. This envelope channel comprises an envelope detector 39, the unipolai output signal of which is supplied through a low-pass lilter 40 to a frequency modulator 41 to control the frequency of a pilot signal oscillator 42. YParticularly with respect to the transmission of signals at low signal to noise ratios, the envelope channel shown in Fig. 2b is preferably constructed in a manner such that the module of the frequency-modulated pilot signal is lower than 3. It should be noted that owing to the unipolar envelope signal the frequency sweep is only unilateral.

An important advantage of the transmission of the envelope signal by means of frequency modulation resides in the fact that then the direct-current component of the envelope signal at the receiver end can be determined unambiguously. When transmitting the envelope signal by means of amplitude modulation this is not possible, already owing 'to normal fading phenomena.

Fig. 3a shows a receiver to be used in conjunction with the transmitter shown in Fig. 2a. The signals obtained from the aerial 43 are supplied through a high-frequency amplifier 44 to a channel for the limited single-sideband signal and to an envelope channel. The limited singlesideband signal which was limited at the transmitter end, is selected by means of a single-sideband filter 4S and supplied subsequently via an amplifier 46 to a limiter 47, which suppresses drastically amplitude variations of the single-sideband signal produced in the transmission path.

The signal envelope transmitted by means of amplitude modulation is selected by means of a filter 48, passing a narrow band of for example 100 to 500 c./s. and then supplied via an amplifier 49 to an amplitude detector 50, from the output of which the signal envelope is obtained via a low-pass lter 51. The signal envelope thus obtained modulates, in an `amplitude modulator 5-2, constructed for example as a vario-losser, the single-sideband signal obtained, if necessary via a simple single-sideband filter 47' from the limiter 47, -in order to obtain a singlesideband signal having both phase modulation and amplitude modulation. This single-sideband signal is supplied, subsequent to ampliiication in an amplifier 53 to a conventional single-sideband demodulator, comprising a modulator stage 55, connected to a local carrier-wave oscillator 54. The detected speech signal passes a speechband lter 56, passing only the speech frequency band of for example 0.3 to 3.4 kc./s. and is supplied via a deemphasis network 57 and an amplifier 58 to a reproducing device 59.

As stated above with reference to the transmitter shown in Fig. 2a, it is advantageous, in order to suppress noise during the speech intervals, to use an amplitude-modulated pilot signal, which has its maximum amplitude in the absence of speech signals. When using such a pilot signal, the modulator 52 in the receiver shown in Fig. 3a, in which the limited single-sideband signal and the signal envelope are united, must, of course, be constructed in a manner such that at an increase in amplitude of the detected signal envelope the output voltage of the modulator 52 decreases.

It is furthermore possible to provide such a bandwidth for the input filter 45 for the single-sideband signal that not only the single-sideband signal but also the pilot signal is passed. Thus the limiter 47 in the receiver is driven during the speech intervals by the pilot signal, which then has the maximum amplitude and not by noise voltages, as Would be the case in the absence of such a pilot signal. In the presence of speech signals the singlesideband signal provides, of course, a full loading of the limiter 47.

Fig. 3b shows the construction of the envelope channel in the receiver to be used in conjunction with the frequency-modulation envelope channel shown in Fig. 2b inthe transmitter. y y

The frequency-modulation envelope channel shown in Fig. 3b comprises an inputflilte'r 60, an amplifier 61 and a limiter 62, as employed usually for the reception of frequency-modulated oscillations. The output signals of theplimiter 62-are supplied for detection to a frequ'ency`4 detector 63 of conventional type. Theenvelope signal then obtained is supplied via a low-pass filter 64, having a cut-off frequency of for example 300 c./s. to 50 c./s. to the amplitude modulator 52, shown in Fig. 3a.

It should be noted herein that with signal to noise ratios of about 10 db and less, wide band frequency modulation is not effective to improve the signal to noise ratio of the incoming signal.

As stated above with reference to the transmitter shown in Figs. 2a and 2b, use is preferably made of a module which may be at the most for example 3 with envelope frequencies of 40 to 100 c./s. Of course, the input filter 60 for the frequency-modulated pilot signal must have a passband adapted to the bandwidth of the incoming signal. Since the signal envelope is unipolar, a bandwidth of the filter 60 equal to the three-fold of the maximum frequency of the signal envelope suices with the use of a module 3.

With the receivers described with reference to Figs. lb, 3a and 3b a slight improvement in the signal to noise ratio of the output signal appeared to be obtainable by connecting a single-sideband Vfilter between the limiters 14 and 47 and the next following modulator stages 15 and 52 respectively.

What is claimed is:

1. A single-sideband communication system comprising a transmitter having means for producing a single sideband signal, a first limiter connected to limit the amplitude of said signal, a single-sideband filter connected to' the output of said limiter to suppress distortion products of the limited signal, and'means for transmitting said limited single-sideband signal, and a receiver having means to receive said transmitted limited single-sideband signal, a single-sideband demodulator, a second limiter, and means connected to apply the received signal to said demodulator through said second limiter.

2. A system as claimed in claim l, in which said transmitter includes a source of a pilot signal, and means connected to apply said pilot signal to said lirst limiter whereby said pilot signal is limited along with said sngle-sideband signal.

3. A system as claimed in claim 1, in which said transmitter includes an amplitude detector connected to the output of said means for producing a single-sideband signal to detect said single-sideband signal and produce the envelope thereof, and means connected to said amplitude detector to transmit said envelope, and in which said receiver includes means connected to receive said envelope and a modulator connected between said demodulator and said second limiter to modulate the limited received signal with said envelope.

4. A system as claimed in claim 3, in which said means to transmit the envelope comprises a pilot signal generator and a modulator connected to said pilot signal generator and to said amplitude detector to modulate said pilot signal with said envelope, and in which said means to receive the envelope comprises a detector connected to receive the modulated pilot signal and adapted to detect said envelope from the pilot signal, and means connecting the output of the last-named detector to an input of said modulator.

5. A system as claimed in claim 4, in which said pilot signal has a frequency lying in a frequency band adjacent to the frequency band of the single-sideband signal.

6. A system as claimed in claim 4, in which said modu- 7 8 ltor is a 'fr'equency modulator, and in which said de- 2,653,221 Carnahan Sept. 22, 1953 tector comprises a limiter and a frequency detector. 2,666,133 Kahn Ian. 12, 1954 2,7 75 References Cited in the le of this patent ,v ZJOS 258 UNITED STATES PATENTS 5 FOREIGN PATENTS V1,721,574 Potter July 23, 1929 2,246,184 Peterson June 17, 1941 460,135 Canada Oct. 4, 1949 

